This invention relates broadly to a gain control circuit for an audio amplifier. More particularly, the invention is a gain control circuit in which the amplifier voltage input is continuously monitored to generate a control voltage and the gain is incrementally varied in response to changes in the control voltage.
It is known in the prior art that the gain of an operational amplifier can be regulated by controlling the amplifier feedback resistance. In some prior art devices, amplifier gain is made to vary continuously with respect to the amplifier input voltage. Such gain control is particularly objectionable in high fidelity systems in that it generates an amplifier wave output having a frequency spectrum which differs from that of the amplifier input.
In other prior art devices, the amplifier voltage input may be periodically sampled to provide a resistance control signal. In audio preamplifier systems, it is preferable to continuously monitor the amplifier input and regulate the amplifier gain accordingly. By continuously monitoring the voltage input, the gain control circuit deals with transients that may go undetected by a periodic sampling system, and thus create undesirable output spikes.
In yet another prior art device, the amplifier gain is adjusted by selectively switching a plurality of resistors connected in parallel between the input and output of the amplifier. In this device, the gain is preset to provide an output signal of a predetermined magnitude for an input signal which occurs during the presetting or adjustment period. This device is particularly inapplicable in the audio amplifier environment where it is desired to maintain a constant magnitude output in response to continuously varying input magnitudes.
The present invention overcomes the disadvantages of the prior art gain control systems in that it is a circuit that continuously monitors the amplifier input voltage and accordingly adjusts the amplifier gain in discrete increments. The device is particularly useful in high fidelity audio amplifier equipment where it is desired to maintain a constant magnitude output in response to varying inputs while simultaneously preserving the frequency characteristics of the input signal.